The charging function of a Universal Serial Bus (USB) is to charge a terminal through a USB port of a Personal Computer (PC) or laptop. At present, many handheld devices such as MP3, mobile phone and the like all are provided with a USB charger; a data line, as a power line, is inserted into the USB port of the PC or laptop to automatically charge a terminal, which makes people's life much more convenient. The charging process includes three charging stages: constant voltage charging, constant current charging and tiny-current charging.
Generally, the output voltage from the USB port of a PC or laptop is 5V, which is relatively precise; however, the output current from the USB port is small and a tiny-current charging is needed at the final stage of the charging process. The tiny-current charging is to compensate the capacity loss caused by self-discharge of a battery after the battery is fully charged. After a battery is fully charged, the capacity loss caused by self-discharge generally is 5% of the nominal capacity. Theoretically, the capacity loss caused by self-discharge can be compensated by continuous charging of C/500 current. However, since the tiny-current is too small, the tiny-current charging process is very slow, which then prolongs the charging time and reduces the charging efficiency.
FIG. 1 is a schematic circuit diagram of charging through a USB interface in the related art; in actual applications, during the charging process of electronic devices, generally current is limited to 400 mA, total power input is 2 W, the voltage output from a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) is lowered to a certain voltage range by the battery of the electronic device, generally the voltage range is between 3.6V and 4.3V, at this time the power output is between 1.44 W and 1.72 W, and the current is lower than 400 mA when the voltage is close to 4.3V; therefore, during the tiny-current charging process, the MOSFET used for limiting current consumes power; how to improve the charging efficiency of the tiny-current charging stage is a problem to be solved.